CAP for Mobility Support

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1 CAP for Mobility Support Yuanjie Li 1, Zengwen Yuan 1, Chunyi Peng 2, Songwu Lu 1 1 University of California, Los Angeles 2 The Ohio State University

start at low ber of are e rich New Mobile Services in Future 5G Massive-connectivity machine type communication service requires the underlying future 5G communication system to be capable of

well as hout eing y the elong to this category is expected to be besttively small amount, this service requires the G communication system to be capable of umber of potentially simultaneous control

2 start at low ber of are e rich New Mobile Services in Future 5G Massive-connectivity machine type communication service requires the underlying future 5G communication system to be capable of handling massive number of potentially simultaneous connectivity. well as hout eing y the elong to this category is expected to be besttively small amount, this service requires the G communication system to be capable of umber of potentially simultaneous control plane Critical Machine Type nication [ST3] Real-time virtual reality used he ged for bility d over n., olling, ks, of e nd receive data and control information ge metro areas reliably with minimal end-to-end uture mobile network an ideal network solution, Some example services in this service type are: Smart city, home, and life Smart metering Smart environment management Smart wearables Remote mobile healthcare Emergency communication Mission-critical machine type communication service type requires the underlying communication system to be highly reliable and available even in the case of large scale natural disasters. Some example services are: Remote machinery, vehicle, drone controlling Smart ehealth Public safety networks Tactile Internet Mission-critical communication Safe autonomous driving 2 High-speed mobility

3 Key Enablers for 5G Mobile Services Advanced wireless technologies Advanced mobility support 3

Wide-Area Mobility via Cellular Networks Cellular Networks (e.g.

as the surroundings without human intervention.

5G communication system to be capable of handling massive number of potentially simultaneous control plane connectivity. 3.

4 Wide-Area Mobility via Cellular Networks Cellular Networks (e.g., 3G/4G) As the technologies become mature and start to become available at low cost, increased number of devices and things are connected to provide rich information about individual users as well as the surroundings without human intervention. Although the data being sent and received by the individual devices belong to this category is expected to be besteffort basis and relatively small amount, this service requires the underlying future 5G communication system to be capable of handling massive number of potentially simultaneous control plane connectivity Mission-Critical Machine Type Communication [ST3] As the technologies used to implement mobile networks and how the networks are managed for high performance, reliability, and availability are better understood over time, various mission critical services (e.g., remote machinery, vehicle, drone controlling, public safety networks, etc.) start to be implemented on top of mobile networks. The capability to send and receive data and control information wirelessly across large metro areas reliably with minimal end-to-end... Massive-connectivity machine type communication service requires the underlying future 5G communication system to be capable of handling massive number of potentially simultaneous connectivity. Some example services in this service type are: Smart city, home, and life Smart metering Smart environment management Smart wearables Mission-critical machine type communication service type requires the underlying communication system to be highly reliable and available even in the case of large scale natural disasters. Some example services are: Remote machinery, vehicle, drone controlling Smart ehealth Public safety networks Tactile Internet 4

5 How does Mobility Support Work in 4G LTE? Span on multiple network nodes Involve multiple control procedures Internet Routing update Data-plane Control-plane Session state transfer Location update request User profile update Tracking Area 1 Tracking Area 2 Location update accept Radio conn. setup Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 5

6 Three Expectations for Mobility Support Immediate data service Once route is connected Correct data service Forward packets under right policies Continuous data service Even in presence of failures Can we always achieve all of them? 6

7 This talk A formal analysis of fundamental limits in mobility support From the distributed system perspective Fundamental limits: CAP theorem on generic mobility support Rethink the tradeoff between different properties Argument: 5G needs a paradigm shift in its mobility support 7

8 Outline Q1: What properties do we want? Q2: Can we achieve all properties simultaneously? Q3: How to balance different properties? 8

9 What properties do we want? From mobile user demands to formalization 9

10 What Properties do We Want? Correctness: Forward every packet under correct policies Internet Data-plane Control-plane Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 10

11 What Properties do We Want? Correctness: Forward every packet under correct policies 4G LTE: correctness via sequential consistency Internet Routing update Data-plane Control-plane Session state transfer Location update request Base station (enb) Gateway (SGW/PGW) User profile update Tracking Area 1 Tracking Area 2 Location update accept Mobility controller (MME) User profile database (HSS) Radio conn. setup

12 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) What Properties do We Want? Correctness: Forward every packet under correct policies 4G LTE: correctness via sequential consistency Session state transfer Radio conn. setup Location update request Routing update User profile update Location update accept

13 What Properties do We Want? Correctness: Forward every packet under correct policies Availability: start data service once route is connected Internet Data-plane Control-plane Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 13

14 What Properties do We Want? Correctness: Forward every packet under correct policies Availability: start data service once route is connected Partition tolerance: continuous operation once route is connected Internet Data-plane Control-plane Session state transfer Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 14

15 Can we achieve all properties? CAP theorem on mobility support 15

16 CAP Theorem for Mobility For any mobility support, it is impossible to always guarantee sequential consistency, availability, and control-plane partition tolerance simultaneously. 16

17 Example: Control-Plane Partition Between 4G Tracking Areas Internet Session state transfer Data-plane Control-plane How should I operate now? Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 17

18 Option 1 (4G LTE): Consistency Over Availability Suspend data service until the partition is recovered Guaranteed correctness Internet Data-plane Control-plane Session state transfer Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 18

19 Option 1 (4G LTE): How Much Delay in Reality? Comparable to radio latency L: 193.8ms-6.4s ( %) Suspension (ms) 2,000 1,500 1, Total suspension Routing update State transfer (ctrl ctrl) Radio conn. setup Normalized sorted sample (%) 19

20 Option 1 (4G LTE): How Much Delay in Reality? Suspension (ms) Comparable to radio latency L: 193.8ms-6.4s ( %) Failure recovery incurs extra delay L: 500ms-30s 2,000 1,500 1, Total suspension Routing update State transfer (ctrl ctrl) Radio conn. setup Normalized sorted sample (%) Normal data suspension (s) T-Mobile AT&T Sprint Project-Fi worse Extra suspension for failure recovery (s) 20

21 Option 2: Availability Over Consistency Delivery user data without latest data session states No guaranteed correctness: sequential consistency violated Internet?? Data-plane Control-plane Session state transfer?? Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 21

22 Summary: Impact of CAP Theorem Consistency over Availability Guaranteed correctness J Long data service delay L Availability over Consistency Correctness unguaranteed Immediate data service J 22

23 How to Balance Properties? Between availability, consistency and fault tolerance 23

24 Correctness, Not Sequential Consistency! Sequential consistency is more than necessary for correctness Availability Correctness Partition Tolerance More flexible consistency model to achieve all! Sequential Consistency 24

25 What Consistency is Required for Correctness? Function 1: Data forwarding to correct location Only a subset of network nodes need latest user location Internet Data-plane Control-plane Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 25

26 What Consistency is Required for Correctness? Function 2: Volume-based data billing Decouple packet counting and billing policy Packet counting can be performed together with data forwarding Internet ? Data-plane Control-plane Tracking Area 1 Tracking Area 2 Base station (enb) Gateway (SGW/PGW) Mobility controller (MME) User profile database (HSS) 26

27 Other Cellular Functions in Paper Radio access control Cryptography over the air QoS policy 27

28 Future Directions What is the minimal consistency model for mobility support? Can we have a continuous (thus tunable) balancing mechanism? Will unreliable wireless connectivity affect the tradeoffs? 28

29 Conclusion No free lunch for perfect mobility support Sequential consistency is more than necessary for correctness More flexible balance between C, A and P can benefits 5G 29

30 Backup 30

31 What Consistency is Required for Correctness? Function 3: Radio access control Observation: per-group access control in 4G LTE No need to wait for per-device radio access control list 4G Radio Access Control List Forbidden Tracking Area List (TS24.301) Forbidden PLMN list (TS24.301) Closed subscriber group (TS29.281) Granularity Per tracking area Per roaming network Per cell (private Femtocell) 31

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